Model 29b

DESCRIPTIVE MODEL OF OLYMPIC DAM Cu-U-Au By Dennis P. Cox

DESCRIPTION Hematite, bornite, and other minerals in sedimentary breccia filling grabens in granitic basement.

GENERAL REFERENCE Roberts and Hudson (1983).

GEOLOGICAL ENVIRONMENT

Rock Types Proterozoic alkali granite with red K-feldspar, brecciated and forming clasts in matrix-rich breccia. Felsic volcanic breccia and tuff. Hematite iron-formation.

Textures Granophyric intergrowth in granite. Breccias grade from clast-supported in interior of basin to matrix-supported in central iron-rich part.

Age Range The only example is 1,500 my. old.

Depositional Environment Proterozoic granite basement broken by a deep, narrow graben filled by rapidly deposited breccia, iron-formation, and minor felsic volcanic rocks.

Tectonic Setting(s) Narrow graben transcurrent to broad arch. Local gravity high caused by dense iron-formation. Trace of graben can be detected in post-ore cover rocks as photo lineaments.

Associated Deposit Types Sediment-hosted copper deposits, iron-formation.

DEPOSIT DESCRIPTION

Mineralogy Stratabound hematite + bornite + chalcopyrite; transgressive hematite + chalcocite + bornite with fluorite, barite, and minor carrollite, cobaltite, native silver, coffinite, brannerite, bastnaesite, and florencite.

Texture/Structure Ore minerals in breccia matrix and in veins. Pisolitic siderite-fluorite- chlorite in stratabound ore.

Alteration Hematite-chlorite and sericite-quartz, also carbonates, fluorite, barite, rutile, and rare anhydrite, tourmaline, and magnetite. Intense chlorite alteration of granite below ore bodies. K-feldspar replaced by chlorite.

Ore Controls Stratiform ore in matrix polymictic-breccia containing clasts of granite, pisolitic rock, hematite, and sulfides. Transgressive ore in fractures parallel to long axis of graben.

Weathering Type example not exposed.

Geochemical and Geophysical Signature Cu+U+Co +Au+Ag+light REE+F+Ba. Dispersion pattern not known. Cu associated with hematite. Co associated with lower pyrite-rich zone. U-REE associated with Cu but Cu not always with U-REE. Au highest in late chalcocite ore. Pb, Zn very low.

Radioactivity would be detectable if exposed or shallow. Magnetic high of unknown origin.

EXAMPLES Olympic Dam, AUSA (Roberts and Hudson, 1983)

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